So, I am using a 12V supply to power the Arduino MEGA2560 with an Ethernet shield. I noticed that the onboard regulator on the mega gets very hot to the point it cannot be touched. Is it normal for it to get this hot? Is this heat going to damage the boards? I am planning to use the mega 5V out to power some additional electronics which I think will use 100ma to 200ma in addition to what the mega and the Ethernet shield draw. I am not sure how much heat will be generated from the regulator and if this will damage the boards. I experimented with different supply voltages from 5.5V to 12 V and found that anything above 8V will generate too much heat to the point the regulator cannot be touched. But since I need 12V to power other circuits, I would like to avoid the scenario where I will need to use another source just for the Arduino and the Sheild. Any comments?

Yes, I need 12 volt for other stuff, so I would like to stick with just the 12V if possible. But powering up just the Mega and the Ethernet shield from 12V generates too much heat (the regulator and the surrounding area/parts get too hot to touch). But according to the Mega2560 specs, the voltage range can be 7-12V. I think this spec is not completely true. 12V may work powering up just the mega board but not with anything else attached to it. This should be stated somewhere as this may cause a safety hazard.

Ethernet shield from 12V generates too much heat (the regulator and the surrounding area/parts get too hot to touch). But according to the Mega2560 specs, the voltage range can be 7-12V. I think this spec is not completely true. 12V may work powering up just the mega board but not with anything else attached to it. This should be stated somewhere as this may cause a safety hazard.

I wouldn't be keen on running my Arduino gear on 12v, and would be looking at a dual output PSU if 12v was demanded for other purposes. Having said that, you could be over-reacting and a good solution is to stop touching the things that are hot. If it was a serious problem, it would be common knowledge by now. And don't expect it to run cool on a 9v supply either. My Mega setup is too hot to touch with a 9v supply but has been running more or less continuously for weeks.

You might check that your 12v supply really is 12v. I have one labelled 12v but is really 15v, and that would be stretching the friendship with a 5v Arduino.